CHEMICAL NOTES.
NOTE I.—HYDROCYANIC OR PRUSSIC ACID.
Nature of—Strength of different preparations of, English and
foreign—Where found—Tests, preliminary: (1) Odour—(2) Silver—(3)
Prussian blue—(4) Sulphur—(5) Guaiacum—(6) Uranium—(7) Picric
acid—(8) Cupric sulphate—(9) Cobalt chloride—(10) Mercuric
oxide—(11) Peroxide of hydrogen—(12) Mercurous nitrate. Test
apparatus—Salts of hydrocyanic acid: (1) Potassium cyanide—(2)
Mercuric cyanide—(3) Cyanides of the heavy metals—(4) Double
cyanides—(5) Sulphocyanides—Oil of bitter almonds—Antidotes—Fatal
dose—Symptoms—Post-mortem appearances—Drops and minims—Period
after death at which hydrocyanic acid can be discovered—Formic acid
to be tested for—Processes.
_Synonyms._—Cyanhydric or prussic acid, Hydric cyanide, Hydrogen
cyanide, Acidum borussicum, Blausaüre, Berlinerblausaüre.
Formula HCN, _i.e._, a compound of single atoms, of hydrogen, carbon,
and nitrogen, in the proportions by weight of 1 + 12 + 14 = 27.
In its pure state (anhydrous, or free from water), it is a feebly
acid, colourless, mobile liquid, inflammable and very volatile.
Boiling point 24·5° C. Much lighter than water: sp. gr. ·7058. It
has a characteristic overpowering and oppressive odour, resembling
peach-blossom or laurel-water. But the anhydrous acid, from its
volatility and dangerous character is rarely seen or made. In commerce
it is always found as a dilute aqueous solution, the varying strengths
in real HCN being:—
Per cent. HCN.
Pharmacopœia, British, Swiss, America,}
Borussica, London, Norway, } 2
Schräders 1·5
Pharmac. Saxony 1·9
” Austria, Baden, Batavia 2·5
” Edinburgh, Dublin 3·3
Vauquelin’s acid 3·3 to 3·5
Pharmac. Bavaria 4
Scheele’s acid 4 to 5 (rarely 6)
Duflos’s acid 9
French Pharmacopœia 10 to 10·5
Riner’s and Pfaff’s acids 10
Hessian Pharmacopœia 18 to 20
Koller’s 25
Robiquet’s 50
In this country, only Scheele’s, and the British Pharmacopœia (2 per
cent.) acid, are usually met with.
These numbers, however, must be regarded merely as rough approximations
for two reasons; first, on account of the extreme volatility of the
acid—if loosely stoppered, or frequently opened, it rapidly loses
strength—second, both the anhydrous acid and its aqueous solution are
decomposed by light, with formation of a brown matter. This change is
supposed to be retarded by a trace of mineral acid, hence a little
hydrochloric or sulphuric acid is frequently added to the commercial
solution with this object. But the acid may even be _stronger_ than
supposed, as the methods of preparation are somewhat various, and the
one adopted may have been carelessly carried out. Scheele’s acid is
said to be the most popular among medical men; samples of it obtained
from different large firms and examined by the author showed very
irregular strengths, the lowest being 2, and the highest 8 per cent.
The latter was purchased at the shop of a chemist who said he had
made it himself, and could guarantee it was of full strength. He had
evidently made allowance for deterioration. Woodman and Tidy found 16
samples sold in one neighbourhood as B.P. acid to contain 0·6 to 3·2
per cent. of HCN; others have found 0·25 per cent. not infrequent.
It follows that if, in a poison case, a bottle has been found of a
hydrocyanic preparation of a definite name, or even with a certain
strength or dose marked on it, it will not be safe to trust to such
figures without actually determining the amount. In Ball’s trial
(Lewes, 1860), the judge asked whether this variation in strength would
not make the difference between a medicinal and a poisonous dose?
It would not, as the maximum medicinal dose, 4 grains, of even the
abnormally strong (8 per cent.) Scheele’s acid mentioned above, would
only contain 0·32 grain of anhydrous HCN, and it requires at least
half a grain to cause death, while about 1 grain is the usual fatal
quantity. And a medical man would not even give the maximum medicinal
dose as a beginning, and without precaution.
ACIDUM HYDROCYANICUM DILUTUM, PHARMACOPŒIA BRITANNICA.
We shall use the abbreviation “B. P. 2 per cent.” for this acid, which
has the characteristic odour, a sp. gr. of ·997, and a taste “at first
bland and sweet, ultimately pungent and acrid” (Thomson), “hot and
bitter” (Taylor), “cooling, with pungent bitter aftertaste” (Watts).
If pure, it only slightly and transiently reddens litmus; if other
acids have been added to keep it, it may have a stronger reddening
effect. Also, if pure, it leaves no residue on platinum, and gives no
precipitate with barium chloride, but with silver nitrate it gives an
immediate white curdy precipitate of silver cyanide, not blackening
in daylight as the chloride does, soluble in ammonia, insoluble in
dilute, but soluble in hot concentrated nitric acid. It dissolves
mercuric oxide, giving a mercuric cyanide which may be obtained in
white crystals on evaporation. The vapour is said to be more deadly
than the fluid acid. The weaker the acid, the more permanent it is.
Glycerine increases its stability (J. Williams); this might be useful
if suspected substances had to be kept a long time.
_Occurrence._—Hydrocyanic acid itself has never been found as a
natural constituent of the body, although a compound of cyanogen occurs
in the saliva (_see_ Sulphocyanides). Hydrocyanic acid is not formed
during putrefaction, nor by heating organic substances with chemical
reagents at temperatures up to 212° F., as in testing for poisons. The
only way in which it may be generated from animal matter is by heating
with alkalies _to a red heat_;[20] this cannot, of course, happen in
the ordinary process of testing for prussic acid, though it must be
remembered that cyanide might thus be formed in an _ash_ (by burning),
without having been present in the original substance.
It is rather frequent, however, in the vegetable kingdom, and
consequently in a poisoning case the defence often sets up the theory
that it has been ingested in the food (Tawell’s Trial, &c.). It is
necessary, therefore, to examine in what kind of food, and to what
amount, it may be taken.
Its principal source is the seeds, leaves, and flowers, and sometimes
the bark, of most of the species of the sub-orders Amygdaleæ and Pomeæ
of the natural order Rosaceæ. It does not occur in them ready-formed.
There is a substance called _Amygdalin_, a white bitterish crystalline
body, which may be extracted by alcohol from these plants. Amygdalin
when dissolved by itself in water does not produce HCN, and is probably
harmless, but there exists by its side in the plant a species of
ferment called _Emulsin_ or Synaptase, which has the power, when
macerated in water with amygdalin, of breaking up the latter into
glucose (so-called grape-sugar), benzoyl hydride (oil of bitter
almonds), and hydrocyanic acid. In the plant the amygdalin apparently
exists in cells apart from the emulsin, but by crushing in water, or
masticating in the mouth, the change is very rapidly effected. By
long soaking the same result may happen, as in cherry brandy; here
the diluted spirit dissolves the amygdalin, and the emulsin then may
act. But if, in the stomach, the apple-pips or cherry-stones should be
found _whole_, it is almost impossible that the amygdalin should be
decomposed, protected as it is by its horny or stony envelope. Stones
and pips, in fact, pass through the body intact, and are found in the
fæces.
Yet as amygdalin and its decomposition may be much mentioned by the
defence, the following account may be useful.
100 parts of amygdalin yield 6 parts HCN.
It has been found in the species of Rosaceæ given below, generally in
fruit, flowers, leaves, sometimes bark, rarely root.
_Pyrus_ malus (apple pips), domesticus (pear).
_Prunus_ spinosa (sloe), avium (bird cherry), padus (wild service),
Virginiana or serotina (wild black cherry), capricida, insititia
(bullace), domestica (plum, damson, &c.).
_Amygdalus_ communis (almond), Persica (peach), lævis (nectarine).
_Armeniaca_ vulgaris (apricot).
_Cerasus_ communis (cherry), acida, laurocerasus (cherry-laurel),
Lusitanica (Portugal laurel).
_Cydonia_ vulgaris (quince).
_Sorbus_ aucuparia (mountain ash), torminalis, hybrida.
_Cratægus_ oxyacantha (hawthorn, young branches).
_Spiræa_ aruncus, sorbifolia, japonica (not in herbaceous species).
Hydrocyanic acid, ready formed, has been found in the roots of the
bitter and sweet cassava (Jatropha manihot).
If the poisonous dose of the B. P. (2 per cent.) acid be at least 30
minims (Royle’s Mat. Med., Dr. Harley, 6th ed.) the following table
shows the amount of some of the above which is needed.
┌─────────────────────┬──────────┬─────────┬────────────┬────────────┐
│ │Percentage│Equal to │Amount │ │
│ Substance. │ of │ HCN │required for│ Observer. │
│ │amygdalin.│per cent.│poisonous │ │
│ │ │ │ dose. │ │
├─────────────────────┼──────────┼─────────┼────────────┼────────────+
│Cherry kernels │ 3 │ 0·18 │ 333 grains│Gieseler. │
│Pips of sweet apples │ 0·45 │ 0·027 │ 2222 ” │C.G. Stewart│
│Pips of bitter apples│ 0·85 │ 0·051 │ 1176 ” │ ” │
│Wild service kernels │ 1·5 │ 0·08 │ 750 ” │Hermann. │
│Flowers, fruit, and }│ 1·0 │ 0·06 │ 1000 ” │Riegel. │
│bark of do. }│ │ │ │ │
│Bitter almond pulp │ 4·25 │ 0·25 │ 240 ” │Allen. │
│Sweet Cassava │ │ 0·017 │ 3500 ” │Francis. │
│Bitter do. │ │ 0·027 │ 2222 ” │ ” │
└─────────────────────┴──────────┴─────────┴────────────┴────────────┘
Sweet almonds contain emulsin, but no amygdalin, hence give no HCN
(_see_ Tawell’s Trial, p. 40).
According to my own experiments, 837 _sweet_ apples (apples weighing
135 pounds, pips about 5 oz.), would be required for a poisonous dose
of HCN; whereas 130 _bitter_ apples, weighing 18 pounds, and the pips
about 2½ oz., would suffice. The pips of bitter apples are bigger, more
numerous, and weigh about three times as much as those of sweet apples.
Among substances containing much more HCN, and actually poisonous on
that account, are:—
HCN.
Crude bitter almond oil 8 to 15 per cent.
Bitter almond water ¼ to 1 ”
Cherry laurel oil 2 to 3 ”
” ” water[21] ¼ to ¾ ”
Cluster cherry oil 9 to 10 ”
(Allen, Comm. Org. Anal.) It is obvious that of fruits an impossibly
large quantity must be eaten to produce any considerable amount of HCN.
In Tawell’s trial, Mr. Cooper, the analyst, deposed that the seeds from
15 apples gave him an exceedingly small quantity of Prussian blue.
Whereas, Henry Thomas, a druggist’s assistant, stated that “15 small
apples gave 2¼ grains of silver cyanide” [equal to 0·46, or nearly ½
a grain, of anhydrous HCN, corresponding to 25 minims of B. P. acid,
nearly a poisonous dose!] “This was done under the direction of a
lecturer at the London Hospital.” A fair sample of the erroneous and
bewildering evidence that is frequently offered in courts of justice.
Mr. Cooper also stated “there is a great difference between bitter and
sweet apples; the bitter contain a great deal of prussic acid, the
sweet, I believe, none at all!” This statement is misleading; no apples
_contain_ prussic acid, but all that I have met with will _yield_ it
by maceration, as all contain amygdalin. The highest class of eating
apples, such as Newtown pippins, Ribstones, and Blenheims, contain only
a minute trace. These have very few pips, 3 to 5 to each apple, while
the bitter varieties, such as “winesours,” have 9 to 13 pips.
In the arts, cyanides are used in photography, dyeing, cleaning
lace and metals, electro-plating, removing silver stains, &c. Their
solutions may cause accidental poisoning, either by the fumes or by
absorption through the skin, especially if the latter is abraded.
Hydrocyanic acid is also formed (1) in the preparation of nitrous ether
(sweet spirit of nitre), (2) by distilling albumen, fibrin, casein, or
gelatin, with sulphuric acid and bichromate of potash, or manganese
peroxide, (3) by the dry distillation of albuminous bodies. It is
hardly necessary to say that these formations could not occur in the
ordinary methods of testing.
TESTS: PRELIMINARY.—It cannot be too strongly insisted that all
operations for the detection of HCN should be carried out as soon after
death as possible, on account of the loss from volatility, or from
secondary changes. (See Sulphocyanides.)
Allen asserts (Commercial Organic Analysis, 1879), that detection in
the body is rarely possible more than twenty-four hours after death;
but Taylor (Med. Juris., 1873, p. 368) has found it in the stomach
twelve days after, saying, however, that “after the stomach had been
exposed a few days longer, all had disappeared.” In a dog’s stomach he
found it, after twenty-four hours’ exposure, _and washing with water_.
In a human stomach, success was achieved seven days after death, where
no odour was perceptible; in another case, after twenty-two days in
the stomach, and after two months in the spleen. It may be found in
the stomach, and not in the tissues; but in most cases it is easily
detected, soon after death, in the blood, organs, &c. The vapour of
HCN will traverse paper, wet or dry bladder, &c., in a few minutes
(Taylor), and few stoppers are close enough to retain it. Hence care
should be taken to shut up the suspected matters at once in glass
bottles _accurately stoppered_; bad stoppers are worse than corks.
The stomach should be first examined entire, to ascertain _odour_,
&c., _noticing whether alkaline or acid_, then cut in pieces, under
distilled water sufficient to cover it, the whole measured, and
one-half (acidulated with tartaric acid, if alkaline), placed in a
capacious retort, and distilled in a bath of water saturated with salt
to raise the boiling point. The condenser should be well supplied with
cold water, the receiver attached airtight, with a mercury valve (a
narrow glass U-tube, containing mercury), to prevent undue pressure. A
little distilled water, about ½ oz., should be placed in the receiver.
The distillation should be continued till one-third to one-half of the
original liquid has passed over. The tests may then be applied to the
distillate.
Allen recommends us to distil _with water alone_ about one-half. If
there is no result on testing the distillate, continue with addition of
tartaric acid. Finally, add a considerable excess of moderately dilute
sulphuric _and_ hydrochloric acid, and carry the distillation nearly
to dryness. In the last stage sulpho-, ferro-and ferricyanides and
mercuric cyanide are decomposed, and give HCN. The original should be
tested for ferrocyanide, &c. This seems a process calculated to give
the clearest idea of the form in which the HCN is present, but is open
to the objection that it is protracted, and may hence cause loss.
Sokoloff. (Chem. Centr., 1876, 603) advises a much more heroic
treatment. “Strongly acidify with sulphuric acid, and distil over
a water bath for _two or three days_, replenishing the water as
evaporated. The longer the distillation, the more accurate the result.”
He adds, that the muscles contain the greater part of the HCN. He
quotes figures in support of his results, but I have not found such
prolongation necessary; and we must remember that HCN is decomposed by
heating with moderately strong mineral acids.
The following modification, proposed by the author, may be
advantageous, as diminishing the risk of loss, and also effecting
concentration:—Prepare _exactly_ equivalent solutions of silver
nitrate, and hydrochloric acid: the silver solution may contain 17
grammes of silver nitrate, the hydrochloric solution 3·65 grammes
of hydric chloride, per litre. Place in the receiver 100 cubic
centimetres of the silver solution (= 1·70 gramme silver nitrate)
before distillation. This is allowing large excess, to provide for
exceptional quantities of HCN. If any quantity of HCN be present, the
liquid in the receiver will become milky; if it does not, there cannot
be more than a minute trace. Transfer the distillate and washings to
a retort, provided with a thistle-funnel, and boil down to one-third
of its bulk; then add, through the funnel, 100 cubic centimetres of
the hydric chloride solution, which will precipitate all the silver
as chloride, and liberate the HCN. Distill with the same precautions
as before: the first 25 cubic centimetres will contain probably all
the HCN. If doubted, a further quantity may be collected and tested.
The 25 cubic centimetres of distillate may now be subjected to the
following tests, taking care that each portion is _measured_ before
being examined, in order that the idea of the quantity present may be
definite. For instance, in the Prussian blue, and sulphocyanide tests,
the resulting colour may be imitated by standard solutions: in the
silver test, a standard silver solution should also be used, and thus a
triply-confirmed knowledge of the quantity present may be attained; and
little bottles, containing the results, should be preserved, to show in
the courts of justice.
I. ODOUR.—All tests involving odour are affected seriously by the
remarkable differences between different people as to their sense of
smell. We hear much of “colour-blindness;” but the analogous olfactory
defect has almost escaped remark. Yet “smell-blindness,” as I have
formerly christened it, or “anozism,” if a Greek word be required,
is exceedingly common, and chemists and medical men are frequently
afflicted with it. I have known an artist, who could not smell strong
ammonia, yet delighted in the odour of new paint, which he compared
to roses. Many laboratory students can neither smell acetic acid,
arseniuretted hydrogen, nor cyanogen. An assistant was so fond of
sulphuretted hydrogen, that he was once found insensible beside the
apparatus, having narcotized himself with the gas (he recovered);
and many more such eccentricities. In the case of prussic acid these
diversities are enormous. Some are so sensitive, that the least trace
in a room becomes rapidly unbearable, causing headache and nausea;
others are like photographers, and can work in a heavily-cyanogened
atmosphere. Such idiosyncrasies become of great importance in evidence;
for example:—
In Tawell’s trial, Mr. Champneys, surgeon, testified as follows: “Have
no experience in detecting odour of prussic acid in a human subject.
Should think it may be taken without detection. Should expect it in the
mouth and breath, but there may be exceptions. There was no odour in
her [the deceased’s] breath; but, on opening the body, I was positive
I smelt prussic acid. The other two surgeons could not smell it.”
Afterwards, when the contents of the stomach were transferred to a jar,
neither the three surgeons, nor Mr. Cooper, the analyst, could perceive
the least odour of prussic acid, even when the contents were boiled.
Nor was it smelt in the blood. Mr. Cooper subsequently stated: “I have
no doubt that prussic acid may exist without being smelt: absence of
smell may arise from dilution, or from its being covered by the smell
of other substances. When I smell it, it affects spasmodically the back
of the throat. _Sometimes it has produced a spasmodic constriction
about the throat without my smelling it._” Here was a well-marked case
of intermittent smell-blindness.
There were also several questions as to whether prussic acid might
have existed in the form of an inodorous salt. Mr. Champneys further
stated that he put ½ drachm of prussic acid into a tumbler filled
with Guinness’s porter, and the smell was _scarcely perceptible_. Mr.
Norblad, surgeon, deposed that he mixed 12 grains of prussic acid with
a pint of porter, but could not then smell it. “Some of the porter
dropped on the table, and I did then smell it.” In the same trial,
Henry Thomas, druggist’s assistant, mixed 30 drops of B.P. prussic
acid with 11 oz. of porter, and found the odour of the acid _slightly
perceptible_; yet, when he was pouring Scheele’s acid from a bottle,
three women had to leave the room to avoid suffocation!
In a case of suicide by cyanide of potassium (Chem. News, 1861, p.
261), the smell of prussic acid was not perceived by the surgeon,
either immediately after death or at the post-mortem examination,
nor by the analyst until the contents had been distilled with dilute
sulphuric acid.
To help in elucidating this matter I have made some experiments as to
the detection of the odour of prussic acid. An acid of 2 per cent.
strength (B.P.) was used.
1. From a bottle of Guinness’s stout, freshly opened, 3 samples of
1 fluid oz. each were measured. To the first 1 drop of the acid was
added, to the second 2 drops, the third being left untouched. This was
done out of my sight in another room. They were then privately marked
by an assistant, and brought in; when myself and two others, one of
them entirely inexperienced, independently and at once classified them
without hesitation correctly as to the relative _amounts_ of prussic
acid. The odour was so distinct as to produce, when inhaled, a feeling
of oppression, and to _quite overpower the odour of the beer_.
1 drop in 1 fl. oz. = 0·23 per cent. of the dilute (B.P.) acid.
” = ·0046 per cent. of real anhydrous HCN.
About 1/30 of a poisonous dose. Hence if a poisonous dose were put into
a pint and a half of stout, the odour would be distinct.
2. One drop of the dilute (2 per cent.) acid was added to 6 oz.
stout: there resulted a slight but distinct odour of prussic acid.
Hence a poisonous dose in _nine pints_ would be smelt. Covered by a
watch-glass, with a drop of yellow ammonium sulphide on it, it was
warmed; the drop, on evaporation, gave a distinct red sulphocyanide
reaction with ferric chloride. Exposed to the air for twenty-four
hours, all the above samples had lost their odour, and failed to give
the sulphocyanide reaction.
3. Two samples of urine, measuring ½ pint each, were treated
respectively with 1 and 2 drops of B.P. acid (strength in this case
1·18 per cent. HCN), and a third ½ pint left untouched, the same
precautions being used as with the above beers. Three independent
witnesses again classified them without difficulty as to relative
amounts of the poison. This is 2 drops of an exceptionally weak acid to
the pint.
4. The contents of a human stomach, very fetid from decomposition, were
divided into two portions of about 2 oz. each: one was left untouched;
to the other 1 grain of mercuric cyanide was added, and then about
5 drops of hydrochloric acid, and a little zinc dust. The whole was
well stirred, and shut up close. Next day the odour of HCN was very
prominent in the one to which the cyanide had been added, in spite of
the strong original smell of both.
5. I cannot agree with Taylor that either peppermint or tobacco mask
the odour appreciably.
The odour of _nitrobenzol_, being similar to that of bitter almonds,
might lead to a suspicion of prussic acid without due caution (Woodman
and Tidy).
In putrefying, organic matters often develope ammonium sulphide,
becoming alkaline. The ammonium sulphide would combine with the HCN
to form sulphocyanide of ammonium, _which is inodorous_, but, by
distillation with acids, gives HCN. Sulphocyanide, however, could not
be produced _unless the original matters were alkaline_. In Tawell’s,
and most other trials, the stomach contents were _acid_, as they always
are naturally from the gastric juice.
Taylor (Med. Jurisprudence, 1873, vol. i., p. 364) mentions a case
where the blood had a strong odour of prussic acid, and the mucous
membrane of the stomach, _even after it had been washed three times
with water_, also exhaled a strong odour. In another case (Med. Gaz.,
vol. xxxvi., p. 104), where 20 grains of Scheele’s acid had been taken
with ultimate recovery, the vomited matters had _no odour_, “showing
that, if not concealed by other odours, the whole of the acid must
have been absorbed.” Many other instances might be quoted where nothing
was smelt, and yet the tests revealed prussic acid.
As to the question about the _salts_ of prussic acid, it may be
generally said that all poisonous cyanides would smell in the stomach,
except, perhaps, mercuric cyanide. _See_ “Properties of the Salts,”
p. 73. Possibly the formation of mercuric cyanide may have accounted
for the absence of odour in some of the above cases, as I do not find
that mercury was tested for, though its compounds are common medicines.
Otherwise it is hardly possible that hydrocyanic poisoning should
have been effected, and the acid be still there, without its very
characteristic odour being perceptible to an observer with an acute
olfactory sense. I have entered at some length into the question of
odour, as much importance has been attached to it in the trials, and I
still consider it as one of the most delicate and positive of tests.
II. SILVER TEST.—When silver nitrate is added to a solution containing
HCN or a cyanide acidulated with nitric acid, a white precipitate
falls of silver cyanide, soluble in ammonia, insoluble in dilute, but
soluble in hot concentrated nitric acid, and not blackened by light.
This reaction is rendered quantitative according to Liebig’s volumetric
method. The original solution is made slightly alkaline by potash, and
a standard solution containing 1·7 grammes of silver nitrate per litre
(1 cub. centimetre = ·0017 grm. AgNO_{3}) is added until a permanent
white turbidity is produced, seen best over a sheet of black paper
or a black book. Then each cub. centimetre used is equivalent to a
_double_ quantity or ·00054 grammes of HCN. Formic acid, or chlorides,
do not interfere; in fact, it is advantageous to have a little chloride
present.
The silver cyanide may be also estimated _gravimetrically_ by adding
excess of silver nitrate, collecting the precipitate on a weighed
filter, washing, drying, and weighing. Silver cyanide corresponds
to two-tenths of its weight of HCN (134 gives 27). If chlorides be
present, the mixed precipitate of silver cyanide and chloride is
weighed, treated with dilute hydrochloric acid, and weighed again.
The HCN is thus displaced, and passes into the filtrate; the silver
precipitate, now all as chloride, is weighed again: then the increase
of weight multiplied by 27 and divided by 9·5 (the difference between
the equivalent weights of silver chloride and cyanide) is equal to
the _weight_ of HCN present. But the volumetric process is quite as
accurate, and more expeditious.
In poisoning cases advantage is taken of the opacity of silver cyanide
thus: A drop of moderately dilute silver nitrate is placed on a
watch-glass over the substance, which may be gently warmed, taking
care that the steam condensed does not cause the drop to fall. If
HCN be present, the drop will become opaque-white from formation of
silver cyanide. 1/100 grain of HCN, equal to ¼ grain of B.P. acid, will
give this reaction (Taylor). If there is only a small amount, and the
action is gradual, the drop on drying in the air may exhibit crystals
of silver cyanide, recognizable under the microscope as minute prisms
obliquely truncated. Of course the silver nitrate itself may give
crystals, but they will be very soluble in water.
Cyanide of silver is decomposed by (1) hydrochloric acid, giving
silver chloride; (2) dilute sulphuric acid and zinc, giving silver;
(3) sulphuretted hydrogen, giving silver sulphide; in each case HCN is
liberated and may be distilled off: then the other tests may be applied.
If sulphuretted hydrogen be present, it will give a _black_ with silver
nitrate. The liquid should in this case be previously shaken with just
enough carbonate of lead to remove the sulphuretted hydrogen. The
latter, however, does not interfere with the Prussian blue or sulphur
tests.
When sufficient in quantity, the cyanide of silver, thoroughly dried
in a water-bath, may be transferred to a small bulb-tube and heated,
the end being closed with the finger. It breaks up into cyanogen gas,
silver, and paracyanide of silver, a peculiar glow and effervescence
occurring as it decomposes. The cyanogen will have the characteristic
bitter almond odour, and, on removing the finger, will burn with a
flame violet on the margin and rosy in the centre.
III. PRUSSIAN BLUE (Scheele).—Add to the solution or distillate
caustic potash in excess, then a drop or two of fresh ferrous sulphate
(protosulphate of iron), and a little ferric chloride (perchloride
of iron—the tinct. ferri perchlor. of the Pharmacopœia will do),
warm gently for a few minutes, add dilute hydrochloric acid in slight
excess: if much HCN be present, a deep blue precipitate (Prussian blue)
will remain; if only a trace, the liquid will be greenish, and on
standing till the next day a blue deposit will form.[22] This is the
only _blue_ iron precipitate which is insoluble in dilute hydrochloric
acid.[23]
_Remarks._—Sulphuretted hydrogen does not interfere with this test,
as the black ferrous sulphide dissolves in hydrochloric acid. The
amount of iron salts added should have some relation to the amount
of HCN present, an idea of which will have been attained by the
silver test. Moderate excess of potash must be present all the time
till the hydrochloric acid is added. A large amount of iron salt is
objectionable, as the yellow colour interferes with the final green
tint with traces of HCN. The test may be made quantitative by imitating
the tint with a weak standard solution of potassium ferrocyanide
treated with a drop of hydrochloric acid and a drop of ferric chloride,
on the same principle as “_Nesslerizing_” (see Wanklyn’s Water
Analysis). Finally the precipitate of Prussian blue should be preserved
to exhibit at the trial, as this is the most positive, though not the
most delicate, test.
IV. SULPHUR TEST (Liebig).—The liquid to be examined is placed in a
somewhat shallow glass dish or beaker, covered almost airtight with
a watch-glass, moistened on the under surface with a drop or two of
_yellow_ ammonium sulphide. [The ordinary sulphide is commonly yellow
enough for the purpose, or, if not, can be made so by warming with a
little flowers of sulphur.] After warming gently for a short time (the
periods recommended by different authorities vary from half a minute to
ten minutes), great care being taken that the steam does not condense
and cause the solution on the watch-glass to drop back into the liquid,
the cover is removed, dried on a water bath to drive off any excess
of ammonium sulphide, treated with a drop or two of water, and a drop
of not too acid ferric chloride free from nitric acid and diluted
till nearly free from colour. If HCN be present, it will have formed
sulphocyanide with the ammonium sulphide, and will therefore generate
a blood-red colour with the ferric chloride. If a colour be produced,
continue the addition of ferric chloride till no further deepening
occurs. The reaction is made quantitative by comparing the tint with
that produced by a known quantity of sulphocyanide and ferric solution
(Herapath). But there are difficulties in making it exact.
This is the most delicate test for HCN, detecting 1/7930th of a
grain of HCN in a very dilute liquid, whereas Prussian blue does not
discover less than 1/780th of a grain (Taylor, Ann. Ch. Pharm. lxv.,
263). Salts of acetic, formic, and meconic acids give red colours with
ferric chloride, but (1) meconic acid is not volatile; (2) the red
from acetic and formic acids is at once removed by a slight excess of
dilute hydrochloric acid, sulphocyanide is not; (3) sulphocyanide-red
is destroyed by solution of mercuric chloride, the others are not.
The above tests are sufficient, but the following additional ones have
been at different times proposed.
V. GUAIACUM TEST.—Paper dipped in fresh tincture of guaiacum,
containing about 3 per cent. of the resin, then dried, then moistened
with dilute cupric sulphate solution (2 per cent.), becomes blue in HCN
vapour. But the same effect is produced _without HCN_ by almost all
oxydants, such as chlorine, bromine, or iodine, ferric chloride, nitric
and nitrous acids, chromic acid, peroxide of hydrogen, ozone (Mohr’s
Toxicologie), also by ammonia, hypochlorous acid, soluble chromates,
&c. (Blyth).
VI. URANIUM TEST.—A grain or two of pure ferrous salt (ammonio-ferrous
sulphate will do), and the same quantity of uranium nitrate, are
dissolved in half an ounce of water. Two or three drops of this are
placed on a white plate, and a drop of the suspected liquid added.
A purple precipitate, or a greyish purple colour in weak solutions,
indicates HCN. Cobalt nitrate may be used instead of the uranium salt,
and is nearly as delicate. (Carey Lea, American J. of Science [3] ix.,
121.)
VII. A hot solution of potassium cyanide mixed with _picric acid_ gives
a deep blood-red —“picrocyanic” acid). Free HCN does not give this
reaction, and therefore must first be neutralized by an alkali. Said to
be more delicate than the iron tests. (C. D. Braun, Zeitschr. f. anal.
Ch. iii., 464.)
VIII. Slightly alkalize the distillate with potash, add a few drops
of _cupric sulphate_, and afterwards just enough hydrochloric acid
to dissolve the excess of cupric hydrate: white cuprous cyanide will
remain undissolved. “This test will detect 1/20000 of HCN in solution.”
(Lassaigne, Ann. de Chimie, xxvii., 200.) But a similar effect is
produced by hydriodic acid, and potassium iodide might have been
administered.
IX. Mix the HCN with excess of alkali, add cobalt chloride and tartaric
acid: on exposure to air a deep brown-red colour will be produced. (C.
D. Braun, loc. cit.).
X. If to a solution of HCN, potash be added in excess, and then a
little very finely pulverised, or precipitated, mercuric oxide, the
latter will dissolve. Mercuric oxide is soluble in alkaline fluids only
in presence of HCN. (Fresenius, Qual. Anal.).
XI. “With peroxide of hydrogen, natural blood gives effervescence
from escape of oxygen, but no discoloration. Blood containing HCN
gives a brown colour, the spectroscopic bands disappearing, and no
effervescence.” (Schönbein.) Hæmatocrystallin, the colouring matter
of the blood corpuscles, combines, in fact, with HCN, giving a dark
coloured compound which appears to be crystallizable and definite in
composition (Hoppe Seyler), does not act as a carrier of oxygen like
the natural hæmatocrystallin, and possesses a distinct spectrum (see
Thudichum, Chem. Physiology). The blue masses in the blood described by
Ralph (Journ. Microsc. Science, Oct. 24, 1866) have not been found by
others.
XII. Mercurous nitrate gives at once with HCN solutions, a black
deposit of metallic mercury, and a solution of mercuric cyanide. With
calomel, a similar reaction takes place according to Allen, but I have
found that the solution is not deodorized even by large excess of
calomel, the odour becoming stronger and more pungent than the original
HCN. On evaporating, mercuric chloride is left. Probably some cyanogen
chloride is formed. The odour is so much intensified that it might be
of use as a test. In view of the possible administration of calomel,
the reaction is interesting.
Of course it will not be necessary to employ all these methods. The
odour, and the silver, Prussian blue, and “sulphur” tests will be
sufficient. I would suggest a form of apparatus by which all the latter
could be obtained from the original substance without distillation in a
retort.
A shallow beaker or glass jar is closed by an india-rubber stopper,
through two holes in which are passed glass rods ending in glass spoon
bowls bent at right angles, so as to be horizontal when mounted. The
bowls should be one inch in diameter, and will have to be specially
made. In the first bowl a few drops of silver nitrate are placed, in
the second a little potash. The apparatus is put in a warm place
for six or eight hours, then the two rods are removed, a third rod
substituted, its bowl containing a drop or two of yellow ammonium
sulphide, the other hole plugged, and the apparatus put back in the
warm place for two or three hours more. The first bowl will have the
silver cyanide, the second should be treated with ferric and ferrous
salt and hydrochloric acid for Prussian blue (_vide_), the third
evaporated and ferric chloride added for the sulphocyanide test. This
arrangement prevents loss of HCN by volatilization, and also, with a
little care, avoids any danger of the reagent dropping back into the
solution. The three rods cannot safely be placed in together, as the
sulphide vapour would blacken the silver.
For the modifications in testing necessitated by the presence of
mercury, &c., see under the different _Salts_.
SALTS.
Hydrocyanic acid combines with bases to form the _cyanides_, which may
be thus grouped:—
A. CYANIDES OF THE ALKALIES (potassium, sodium, ammonium), and of the
ALKALINE EARTHS (barium, strontium, calcium, magnesium). These are all
soluble in water, are alkaline to test paper, and are decomposed by
all acids, even carbonic, hence they _exhale an odour of HCN_, and are
nearly as poisonous as prussic acid itself. If they are present, the
stomach contents must be alkaline. The only member of this group likely
to be met with is
POTASSIUM CYANIDE, KCN. Broken opaque white lumps, or small crystals,
deliquescent, smelling strongly of HCN, soapy to the feel, often
containing much carbonate, and therefore effervescing with acids,
easily fused by heat to a clear liquid, very soluble in water, less
in alcohol. Used for removing silver stains in the form of “cyanogen
soap,” but very dangerous, as a cut or scratch may cause absorption,
and even the unbroken skin, according to Allen, may absorb enough
to cause symptoms. Its aqueous solution decomposes spontaneously
into formiate of potassium, ammonia, and a brown substance. Its taste
is bitter and acrid, causing constriction and a burning heat in the
throat. It is very strongly alkaline. Distilled with dilute acids it
gives off all its HCN. It easily responds to the other tests. In a
case of poisoning investigated by Dr. Bernays, a piece of potassium
cyanide was found in the deceased’s mouth, which was much inflamed by
its acridity. The alkali being strong, and the acid weak, cyanide of
potassium has most of the effects of an alkaline irritant.
The _potassium_ may be found by incinerating a portion of the substance
and testing for it in the ash. Taylor (Med. Jurisprudence) improperly
says that the _salt itself_ (cyanide of potassium) may be recovered
from the organs by incinerating them in close vessels and treating the
ash with water. I have already mentioned that cyanide would be formed
in this way from the organic matters themselves, even if not originally
present.
B. MERCURIC CYANIDE, Hg(CN)_{2}. Of all metals mercury has most
affinity for HCN, mercuric oxide decomposing other cyanides, even
Prussian blue, and dissolving readily, as we have seen, in free HCN,
or in alkaline cyanides. Hence if a compound of mercury have been
given medicinally, the prussic acid will be found in the stomach
as mercuric cyanide, which is easily soluble in water, neutral to
test paper, _quite inodorous_, and extremely poisonous. It is not
officially recognised in any Pharmacopœia, except the French; has
been occasionally used in medicine instead of mercuric chloride,
which it resembles in action, but has the advantage of not being
incompatible with alkalies and organic matters (Royle’s Mat. Med.,
6th ed.). It crystallizes in anhydrous four-sided obliquely-truncated
white opaque prisms, with a disagreeable metallic taste, is permanent
in the air, easily soluble in water, less in alcohol. It fails to
respond to the silver nitrate (partially) or Prussian blue tests,
and gives the sulphur test with difficulty. It is decomposed by
distillation with hydrochloric acid, but only ⅔rds of the HCN pass over
into the distillate, unless ammonium chloride be added (Roscoe and
Schorlemmer’s Chemistry). Whenever HCN is looked for, it is safer to
examine also for mercury, and, if found, to add a little hydrochloric
acid and sulphuretted hydrogen to the original liquid, thereby
precipitating mercuric sulphide (black) and liberating the HCN, which
may be distilled off. If, however, excess of sulphuretted hydrogen
has been inadvertently added, it would blacken silver nitrate, and
hence the silver test would not be available, unless the solution was
previously shaken with lead carbonate to remove the sulphide. But it
would not affect the Prussian blue or sulphur tests, as sulphide of
iron is soluble in hydrochloric acid. Mercuric cyanide also gives off
all its HCN when distilled with iron filings or zinc dust, sulphuric
acid, and water. This seems a better method.
Mercuric cyanide is said to be an irritant poison, and to be similar
in its action to corrosive sublimate. Combination with mercury seems
to mask the physiological action of HCN, just as it does its chemical
action. The medicinal dose is 1/16th grain gradually increased to ½
grain, in pills or solution (Royle). 10 grains have proved fatal. By
heat, when dry, it is broken up like silver cyanide into mercury and
cyanogen.
C. CYANIDES OF THE HEAVY METALS, as zinc, lead, copper, &c. _Silver
cyanide_ has already been described. These are insoluble in water,
inodorous, and probably, while intact, not poisonous. But they are
decomposed by mineral acids, and, as the gastric juice is acid, they
would more or less readily yield free HCN, with its usual odour and
effects. The influence of the metal has also to be considered.
D. DOUBLE CYANIDES, derived from iron, cobalt, manganese, chromium,
platinum, &c., are inodorous. Those of the alkalies and alkaline earths
are alone soluble. The only common ones are ferro-and ferricyanide of
potassium, the so-called yellow and red prussiates of potash. They are
said to be merely purgative, not poisonous, but, from the comparative
facility with which they yield HCN by acids, they cannot be considered
safe. Soluble ferrocyanides give, with pure ferrous sulphate, a white
precipitate turning blue in air; with ferric chloride a precipitate of
Prussian blue; with cupric sulphate a maroon precipitate. Ferricyanide
solutions give with ferrous salts a deep blue precipitate; with ferric
salts a dark-brown coloration. These reactions would be applied to a
filtered portion of the stomach contents. _Prussian blue_ is ferric
ferrocyanide mainly, but varies in composition: it is supposed to be
inert.
Almen states (Chem. Centr. 1872, 439) that potassium ferrocyanide in
solution decomposes at ordinary temperatures, especially if a little
free acid be present, HCN being formed. Prussian blue only decomposes
when warmed to 40° or 50° C. (therefore not in the body, C. G. S.),
“hence the presence of HCN, if accompanied by ferrocyanide, is not a
proof of poisoning.” But ferrocyanide is not in any Pharmacopœia, and
is not administered medicinally. Yet, to answer a possible question, a
known fraction of the original substance might be extracted with water,
and tested as above. The same observations apply to ferricyanide.
When ferro-or ferricyanides are distilled with moderately strong
sulphuric acid, a _portion_ of the contained HCN passes over; in
fact, this is the common process for preparing prussic acid. The iron
remains behind in the retort, in combination with potassium and the
rest of the cyanogen. If ferric hydrate —“ferri peroxidum humidum”),
or ferrous sulphate and potash, have been administered as antidotes to
HCN, Prussian blue might be formed in the stomach. It would then show
a blue colour, either by itself or on addition of an acid, and blue
particles under the microscope, if in sufficient quantity. In this case
the HCN left in the stomach would have been rendered innocuous, and the
prussic acid which had actually caused the death would be found free in
the blood, &c. The stomach contents might then show no HCN, either by
odour or distillation, as Prussian blue is inodorous, and not easily
decomposed by dilute acids. With alkalies it turns brown, giving ferric
hydrate and an alkaline ferrocyanide.
Ludwig and Maushner (Chem. Centr. 1881, 43), in a case of poisoning,
discovered a quantity of potassium ferrocyanide in the body. This was
removed by slightly acidulating and carefully precipitating by ferric
chloride. The filtrate, distilled with tartaric acid, yielded much HCN.
The sample of cyanide of potassium, which had probably caused death,
was afterwards found to contain a large proportion of ferrocyanide.
E. SULPHONCYANIDES (Thiocyanates). Those of the alkalies and alkaline
earths are soluble and colourless; ferric sulphocyanide is soluble,
and intense blood-red (sulphur test); other sulphocyanides are mostly
insoluble. They are all inodorous, poisonous in moderate quantities,
and are not officinal in any Pharmacopœia. Distilled with acids they
break up, HCN being found in the distillate. It has been mentioned
already that ammonium sulphide, produced by putrefaction, may combine
with any HCN present to form ammonium sulphocyanide; therefore, if the
matters to be examined are alkaline, and putrefaction has commenced,
Allen (Commerc. Org. Anal., 1879, art. HCN) recommends us to digest
with alcohol, filter, evaporate to dryness on a water bath, redissolve
in a little water, filter again, and test the filtrate with ferric
chloride after just acidulating with hydrochloric acid: the well-known
blood-red colour will result (_see_ “Sulphur Test”). But the ordinary
distillation with tartaric or sulphuric acid would in this case also
detect the HCN, though the _whole_ might not pass into the distillate.
_Sulphocyanide of mercury_ is the toy called “Pharaoh’s Serpent.” A
case of poisoning by it is recorded.
It is important to notice that traces of sulphocyanide are naturally
present in the saliva. If this salt be found, the question will occur,
how much could be accounted for by the saliva? Carpenter (Princ.
of Human Physiol.) quotes Harley to the effect that the average
daily secretion from the salivary glands is 1 or 2 pounds: other
observers have stated that it varies greatly. The secretion itself
is said to contain, in 1000 parts, one part (Frehrichs), or 0·6 part
(Jacubowitsch), or even 0·3 part (Oehl), of potassium sulphocyanide;
that is, 4·2 to 7 grains per pound, equivalent to from 1 to 2 grains
of HCN, or 2 to 4 grains if 2 pounds of saliva were secreted.
This would be a serious matter but for the fact that, whether from
decomposition by the gastric juice or otherwise, or from its passing
out of the stomach as it passes in, it is certain that no such quantity
is ever found naturally in the stomach, not more than a minute trace
being ever given by the processes, unless hydrocyanic acid, in one of
its forms, has actually been administered.
_Cyanide of cadmium_, and some of its double salts, are sparingly
soluble. _Double cyanide of silver and potassium_ is soluble and
crystallizable. It is the salt used in electro-plating, and,
as commonly met with, smells strongly of potassium cyanide.
_Zinc-potassium cyanide_ has been used medicinally: it occurs in
beautiful crystals, inodorous when dry, but having a faint odour of HCN
in solution.
The other cyanides are rare, and their physiological action is
unrecorded. _Cyanic acid_ and cyanates are said not to be poisonous.
_Oil of Bitter Almonds._—The crude oil contains, as we have seen, 8 to
15 per cent, of HCN. Dissolved in spirit it forms “essence of almonds,”
and is exceedingly poisonous, having caused thirty-one deaths in four
years (Taylor). Two drachms of the oil has killed a man in seventeen
minutes (_Lancet_, 1868, p. 447), two ounces caused death immediately.
The odour of almonds is always distinct in the stomach.
The oil can be freed from HCN, but then does not keep so well, and is
much more costly. Its sp. gr. is 1·049; it boils at 356° F. The crude
oil is yellow: with concentrated sulphuric acid it gives a crimson-red
colour, and on diluting a yellow emulsion. We may estimate the amount
of HCN in it by shaking with water, separating, adding dilute potash
to the aqueous liquid, and testing it with standard silver solution
as described under “Silver Test.” The other tests may also be used
to prove the _presence_ of HCN; the guaiacum and copper paper being
specially convenient.
A case of poisoning by bitter almonds is reported in the “South
Australian Register” for August 6th, 1879. A female child (whose
age is not stated) ate a dozen of them, freshly taken from the tree,
and died in three hours. The symptoms described are pain, coma, and
convulsions.
_Antidotes_ to HCN are generally useless since the death is so
sudden. A moderately dilute solution of an alkali, such as potash,
lime or washing soda, along with a little ferrous sulphate, would
render harmless so much of the poison as was still in the stomach
unabsorbed. As already mentioned, this would cause a little difficulty
in the chemical analysis. _Ammonia_ acts as an antidote by opposing
the _depressant_ action of HCN. _Chlorine water_ has been used: this
converts the HCN into ammonium chloride, carbon monoxide and dioxide,
and a little cyanogen chloride.
_Medicinal uses._—Its primary action is on the cerebrospinal nerves.
It is employed externally, largely diluted, to allay neuralgia and
itching of the skin, and to relieve earache (not more than two drops of
B. P. acid at a time)[24]: it must not come in contact with abrasions,
or it might be absorbed and produce poisoning symptoms. Internally, it
allays dyspepsia and the irritant effects of capsicum, &c. (Royle).
Safe dose internally two to six minims of the B. P. 2 per cent. acid,
suspended if there is any constriction of the throat (Farquharson’s
Therapeutics).
_Fatal dose._—Smallest recorded (Med. Gaz. 35, p. 896); twenty grains
of Scheele’s acid, fatal in twenty minutes, equal to fifty grains of
B. P. 2 per cent. acid, equal to one grain of anhydrous prussic acid.
Largest dose with recovery (_Lancet_, 1854, January 14), _one drachm_
(sixty grains) of Scheele’s acid, but in this case energetic remedies
were at once applied. Average fatal dose of 2 per cent. acid, thirty
minims (Royle’s Mat. Med., Dr. Harley, 6th ed.).
_Symptoms._—These vary with the dose, &c. A large quantity kills in
two to five minutes, though insensibility may ensue in a few seconds.
But patients may survive for twenty minutes, or even for an hour; and
may continue in imminent danger for several hours, and yet recover
(Guy and Ferrier, Forens. Med., 1881). Many cases have occurred of
voluntary acts, such as concealing or throwing away the bottle,
having been performed after fatal doses had been swallowed (_Ibid_,
p. 600). In animals, according to Mr. Nunneley, there is usually a
peculiar plaintive cry, but not in man, though there may be a call for
assistance. Convulsions, and involuntary evacuation of fæces or urine,
may or may not occur. Large doses kill by cardiac syncope; smaller ones
by paralysis of the respiratory centre, or, if gradual, by impeded
oxidation of the blood (Farquharson’s Therapeutics). Other symptoms
are, dilatation of pupils, muscular prostration, deep convulsive
breathing at long intervals, quick feeble irregular pulse, spasmodic
closure of the jaws and clenching of the hands (Taylor). Breathing
sometimes stertorous (Christison, Ed. Month. Journal, February, 1850,
p. 97. Reg. _v._ Burroughs, Cent. Crim. Court, February, 1857).
Vomiting occasional, or foaming at the mouth.
_Post-mortem appearances._—Not characteristic (Farquharson; Guy and
Ferrier). Putrefaction not accelerated (Taylor). The veins contain dark
fluid blood: the right side of the heart is gorged (Harley). There may
or may not be congestion and reddening of stomach and intestines, or of
the brain. On the whole, the appearances are those of asphyxia.[25] The
odour should be sought for in all parts, and _as soon as possible_ the
organs should be shut up in stoppered jars, or well-corked and sealed
bottles, and sent at once for analysis.
The symptoms and post-mortem appearances of poisoning by _Cyanide of
Potassium_ are the same as those of prussic acid, except that:—
1. Convulsions are more common.
2. Owing to the irritant action of the alkali, the stomach is reddened.
3. The contents are alkaline.
The fatal dose is less than five grains, but Taylor mentions a case of
recovery after nearly one ounce of the commercial cyanide, which may,
however, have contained much carbonate.
Hydrocyanic acid is not, in the strict sense, a _cumulative_ poison;
“but doses that exceed the proper medicinal limit may happen to prove
fatal though similar previous ones have appeared to be harmless,
in consequence of a change in the body itself.” (Guy and Ferrier’s
Forensic Medicine, 1881, p. 606.)
In the trial of George Ball for poisoning his mother with prussic
acid, at Lewes, July, 1860 (previously reported), the question arose
as to the difference between _minims_ and _drops_. A minim of water
is supposed to weigh a grain: if the fluid is heavier than water, it
weighs more than a grain; if lighter, it weighs less. But a _drop_ is
quite an indefinite quantity: it is affected, not only by the specific
gravity, but by the cohesion of the fluid, by the shape and size of the
vessel, the manner of pouring, and the temperature. I have made some
experiments which show the irregularity. (See also Woodman and Tidy’s
Forensic Medicine, p. 456.)
┌────────────────────────────────────┬───────────┬────────┬────────┐
│ │ │ │Measured│
│ │ │ No. of │ in │
│ Capacity of Bottle. │ Liquid. │ Drops. │Minims. │
├────────────────────────────────────┼───────────┼────────┼────────┤
│(Stoppered) 6 fluid oz. │ Water │ 117 │ 180 │
│ Do. (another observer)│ Do. │ 90 │ 120 │
│ 1½ fluid oz. │ Do. │ 47 │ 100 │
│(Corked) 6 do. │ Do. │ 36 │ 100 │
│Same capacity, dropped with the cork│ Do. │37 to 41│ 100 │
│ (Stoppered) 6 fluid oz. │ Rectified │ 243 │ 120 │
│ │ Spirit │ │ │
└────────────────────────────────────┴───────────┴────────┴────────┘
Proving that while a drop may be estimated at about 1½ to 2 minims (a
good deal more than the usual supposition, the two terms being often
regarded as synonymous), yet the inconstancy is so great that it is
absolutely imperative, in using powerful medicines, to prescribe exact
measurement, and not such a precarious process as dropping.
As to the period after death during which HCN may be detected, Allen
(Comm. Org. Anal.) asserts that its detection is rarely possible after
more than twenty-four hours. This is astonishing, as Casper separated
more than 18 milligrammes from a body eight days after death; Sokoloff
detected it in hounds sixty days after; Dragendorff after four weeks in
a dog, after eight or ten days in man. Reichardt (Arch. Pharm. 3, 19,
204) found it in a body two months after death—in the organs, but not
in the urine. In the Tawell trial, also, the interval was considerable.
Casper states in his Handbook (vol. iii., illustrative cases of HCN)
that Schauenstein (one of the Prussian official chemists), twenty-six
hours after death, found no HCN in the stomach, but a considerable
amount of _formic acid_, the result of its metamorphosis. We know that
strong HCN, exposed to light, decomposes into formate of ammonium,
which, by distillation with a dilute acid, would give formic acid in
the distillate. That such a change should occur so rapidly in a dilute
solution, and in the darkness of the body, is improbable. It would be
well, however, that formic acid should be looked for in the distillate
thus:—
Carefully neutralize a _measured portion_ with pure soda or potash;
evaporate on the water-bath to dryness. The alkaline formate will be
left in white crystals if present, together with the cyanide, which
will not crystallize, but remain as a deliquescent mass. Dissolve in a
little water, and divide into three equal portions.
(1.) To the first add silver nitrate in slight excess. Cyanide of
silver will precipitate, formate will remain in solution, if not too
concentrated. Filter, if possible. On boiling, if any blackening
happens from reduction of the silver, formic acid is _probably_
present. _Acetic_ acid does not reduce silver nitrate.
(2.) To the second add dilute _neutral_ ferric chloride (a solution of
iron-alum answers admirably). A red-brown colour, removed by a drop of
hydrochloric acid, indicates either acetic or formic acid.
(3.) Evaporate the third portion to dryness, and ignite gently in a
closed crucible. Formate and acetate will be turned into carbonate,
while cyanide will remain unchanged if air be excluded. If then
effervescence take place on treating the residue with a little
hydrochloric acid, it is a confirmation of the presence of formic or
acetic acid. The first test will have revealed which it is.
Many animal substances, when distilled with strong acids, _do_ give
acetic and formic acids, but they do not act thus with _dilute_ acids.
Yet a stomach will usually yield a little acetic acid from the food
having turned sour.
If formic acid be present, it will probably have proceeded from the
decomposition of HCN. Then the reduced silver obtained in the first
test should be weighed, and calculated into formic acid, and also into
hydrocyanic acid (108 parts of silver = 46 parts formic acid, or 27
parts HCN). The result may be stated thus:—
“Hydrocyanic acid actually found,—-- grains. Formic acid found,—--
grains. If this had proceeded from the decomposition of hydrocyanic
acid, it would correspond to an additional amount of—-- grains of
hydrocyanic acid.”
It is needless to observe that the mere finding of formic acid would
be no proof of the administration of HCN, unless strong corroborative
evidence were at hand.
On the whole, we must always try, and we may often hope, to find HCN if
given, either free, as cyanide, or as sulphocyanide, even after months
have elapsed.